The polyomaviruses comprise a class of small DNA tumor viruses within the papovavirus group of DNA viruses. Members of the polyomavirus class include polyomavirus (Py) of mice, simian virus 40 of monkeys, hamster papovavirus, and JC and BK viruses of humans. Of these viruses, Py has been most thoroughly characterized with respect to the genetic elements and proteins involved in oncogenic transformation of mammalian cells. Oncogenic transformation of rodent cells by Py requires the continued expression of the Py-encoded middle tumor antigen (MTAg). The MTAg is a membrane-associated phosphoprotein with an associated tyrosine-specific protein kinase activity that has been demonstrated to be, at least in part, a property of the c- src gene product, pp60c-src. The importance of MTAg-associated tyrosine-specific protein kinase activity is suggested by the finding that all known transformation-component strains of Py encode MTAg molecules which possess this associated activity.